A 12 \mathrm{~kg} block moving east at 4 \mathrm{~m} / \mathrm{s} collides head on with a 6 \mathrm{~kg} block that is moving west at 2 \mathrm{~m} / \mathrm{s}. The two blocks move together after the collision. What is the loss in kinetic energy in this collision?
Answer Choices
A. 36 \mathrm{~J}
B. 48 \mathrm{~J}
C. 60 \mathrm{~J}
D. 72 \mathrm{~J}
E. 96 \mathrm{~J}